Purpose: In Japan, high school students who seek employment can apply to only one company at a time at the school's recommendation, and cannot decline a job offer. This shortens the time available for helping students' career selection process, resulting in problems related to effective support and students' consent. Nonetheless, the actual support process and students' cognitive processes have not been examined. The purpose of this study is to elucidate these processes. Design: In this study, we adopted semi-structured interview data collection and the modified grounded theory approach (M-GTA) for qualitative analysis. Method: Participants were 31 Japanese high school students who had decided where to work. Procedure: The constant comparative method and determining theoretical saturation. Result: The analysis generated three core categories, nine categories, five subcategories, and 32 concepts. 'High school students' place-of-employment, decision-making process, and others' support process' had a structure in which three processes coexisted: (i) changes in internal orientation of students; (ii) decision-making with help from parents; and (iii) job-readiness progress with help from teachers. The second and third processes had different effects on the first process. Conclusion: This study presents a theory that predicts and explains a 'career selection process integrated with others' which involves multiple people and is not performed by a single person. Further, this study presents guidelines for practice which differ from conventional career education. It is not realistic to ask high school students for objectives such as self-understanding, life planning, and self-concept formation, or motivations such as interest-oriented, self-realization, and contribution to others. Teachers are required to do their best within the scope of what they and their students can do. The result diagram generated in this study is useful for this purpose.